3 Answers
3

Judo's groundwork (newaza) looks strange to someone from a Brazilian jiujitsu, wrestling, or SAMBO background. Its approach is fairly unique to this particular combat sport. Why? Because rule-sets determine tactics.

The basics of judo newaza

Other than throwing the opponent, one can win in judo by pins, arm-locks that attack the elbow, and chokes. Some players get very good at applying techniques within that narrow range. Olympic judoka who have particularly notable groundwork include Flavio Canto, Jimmy Pedro, AnnMaria Burns (now AnnMaria de Mars), and her daughter Ronda Rousey.

The other significant rule is that the match will be paused and restarted standing if there is a lack of progress in groundwork. Application of this rule varies widely across different referees and different levels of competition. For some referees a stand-up could be triggered by five seconds of no movement, despite a choke or technique being applied. For others, up to twelve seconds of inaction is acceptable, and significant attempts at turnovers, pins, chokes, armlocks, or transitions are enough to allow groundwork to continue.

Anti-groundwork biases

There are a number of reasons that groundwork is frequently nowhere to be seen in judo matches at all levels.

Many modern referees allow very little time for groundwork before standing the players up to restart the match standing. Therefore, many elite judoka simply prefer throws to groundwork. It's common for these players to work for the referee's stand-up instead of a choke or armlock, except for one or two scenarios for which they have techniques they are very confident in.

Many players also focus their judo training time on throws instead of groundwork. This means they might eschew groundwork simply because they might end up in a bad position against a player better versed in groundwork. Because the rules allow for stalling on the ground, which leads to a referee's intervention, this is a successful strategy.

It's also common for judoka to view groundwork as more tiring or time-consuming than stand-up work. Many players avoid groundwork for that reason.

Outlawed and uncommon submissions

From the perspective of modern grappling in the vein of Abu Dhabi Combat Club or NAGA, more submissions are illegal than legal:

All small-joint manipulations are illegal, including finger locks, wrist locks.

Bicep crushes are illegal.

Both blood chokes and air chokes (attacking the arteries and attacking the trachea) are legal, but neck cranks are not. In addition, one cannot touch the "mask of the face" of one's opponent, so smothering and many wrestling-style cross-faces are illegal. The triangle choke is explicitly allowed and taught in judo (as the sangaku-jime). However, some referees will disqualify a BJJ-style triangle choke finish where the head is pulled down, on the grounds that the neck is being strained.

Shoulder locks are not allowed, but since there is a good deal of overlap between elbow locks and shoulder locks, and it's tough to tell as a third party which one is being affected, they are often allowed by referees. All forms of "arm entanglement" locks (ude garame) are explicitly allowed and taught in judo, making Americanas, key locks, and Kimuras legal. "Chicken wings", where the arm is forced up the back in a Kimura-like fashion, are not legal, since they more obviously attack the shoulder.

Straight elbow locks are the most popular submission in judo, and are definitely legal.

Back mount is a particularly interesting case. Since there are no points awarded for hooks, and there is intense time pressure to advance to a pin, choke, or armlock, the calculus for back mount is decidedly different than it is under Brazilian jiu-jitsu or mixed-martial-arts rules. Instead of holding the superior position and waiting for a good choke, it is generally a better strategy to explosively attempt an armbar or collar choke. This is because naked chokes often take longer to set up in the gi, it doesn't help the judoka to stay on the back, and there's virtually no down side to losing the position if the submission attempt fails. Other players, instead of going for the hail-Mary choke, will go for a turnover to a pin, or simply stand up.

a solid post, but i really have to disagree with : "It's also common for judoka to view groundwork as more tiring or time-consuming than stand-up work. Many players avoid groundwork for that reason." i don't know anyone who thinks it's more tiring, it really is just the lack of time allowed that deters most judoka from engaging in ground work. And the BJJ style triangle is ONLY called for being illegal if you pull down on the head. lots of bjj people can finish with out that.
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PatriciaAug 1 '12 at 12:49

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@Patricia Here's one to view groundwork as more tiring. Depending on the situation of course. If I know that the opponent is better at ground than in standing position, I'd rather leave the ground.
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user1221Oct 12 '13 at 9:16

Chokes are allowed, and elbow attacks are allowed. Everything else is not allowed.

There is a bit of a grey area when it comes to "key locks" or ude garami, which can put the shoulder in peril, but it also attacks the elbow so is allowed, as well as the guillotine choke which while a choke can also be a neck crank, so some referees will prohibit it.

Most of the time chokes are abandon because ground work progress has to be pretty much instantaneous, so if the defending judoka can keep his/her chin down for 2 or 3 seconds, matte (stop) is called and they are stood back up.

The reason why backmount is very often abandoned, is because in judo, you only have a very little time to get a pin or submission.

Basically, if the situation isn't making progress in about 5 seconds, competitors are commanded to stand up, and continue the match in tachiwaza, standing position.

For this, the rear naked choke is a pretty rare submission in judo, because you don't have much time, and if the opponent doesn't tap out, the match will continue in standing position.

Basically, you're allowed to make arm locks to shoulder or elbow, or even in between. As said, it is very hard for a third party (referee) to know the exact part of arm where the lock is placed. No wrist locks, or any kind of leglocks.

You are allowed to do any kind of chokes, not neck cranks, and a guillotine from head is a neck crank, but if you have an arm in between, it's counted as a choke.

Because you won't get any kind of points from ground positions such as mount, competitors forfeit the situation if they do not see a straight opportunity for a score.

Update

As @Dave pointed out, you will get points from positions like mount if you're able to hold it for atleast 10 seconds, but not as easily as in BJJ.

Would +1 for the first 3 paragraphs, but mount does score points, starting at 10 seconds. The only locks that are allowed are those that attack the elbow. Shoulder locks are only allowed if they could reasonably be seen as (or cannot be distinguished from) elbow locks, such as in the case of some forms of ude garami.
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Dave LiepmannJul 7 '13 at 20:09

@DaveLiepmann Sorry for the late response. Yeah, mount does score points starting from 10 seconds, as it's a pin. You're right about the locks, but basically any lock allowed in judo can do damage on the shoulder if executed poorly. I'm updating my answer about the mount, I meant you don't get points the same way from it like in BJJ.
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user1221Oct 12 '13 at 9:11